UNIVERSITY  OF   CALIFOBNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


A  Survey  of  Orchard  Practices  in  the  Citrus 
Industry  of  Southern  California 


BY 

ROLAND  S.  VAILE 


BULLETIN  No.  374 

January,  1924 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1924 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://www.archive.org/details/surveyoforcha3741924vail 


A  SURVEY  OF  ORCHARD  PRACTICES  IN  THE 
CITRUS  INDUSTRY  OF  SOUTHERN  CALIFORNIA* 

By  EOLAND  S.  VAILE 


CONTENTS 

PAGE 

Foreword 4 

Introduction 4 

Purpose  of  bulletin : 4 

Importance  and  growth  of  citrus  industry 4 

Collection  of  data. 5 

Factors  upon  which  information  was  gathered 6 

Analysis  of  the  data 7 

Relation  of  citrus  yields  to  environment  and  orchard  practices 7 

Climatic  zones  and  their  effects  on  orchard  practices  and  yields 7 

Effect  of  abnormal  weather  conditions  on  yields 9 

Effect  of  soils  on  yields 10 

Age  of  trees,  and  yields 11 

Fertilization  and  its  effects  on  yields 14 

Yields  as  affected  by  nitrogen  applications 14 

The  use  of  bulky  organic  manures,  and  yields 17 

Effects  of  cover  crops  on  fertilizer  requirements 19 

Irrigation  and  its  effects  on  yields 21 

Amount  of  water  to  apply 21 

Frequency  at  which  to  irrigate 23 

Plowing  and  its  effect  on  yields  23 

The  "limit  of  profitable  cultivation'' 24 

Cost  of  fruit  per  pound 25 

Net  returns  per  acre 26 

Net  returns  on  the  investment 27 

The  most  profitable  use  of  nitrogen 27 

Cost  of  developing  citrus  orchards 30 

Conclusions : 32 

Appendix 33 

Homogeneity  and  accuracy  of  the  data 33 

Methods  used  in  analysis 34 

Classification  of  the  variables 35 

Association  of  variables 36 

Averages  based  on  law  of  large  numbers ". 37 

Partial  correlation  coefficients 37 

Method  of  combining  groves  from  three  climatic  zones  39 

Diminishing  physical  returns  with  increased  use  of  fertilizer 40 

*  Paper  No.  114,  University  of  California,  Graduate  School  of  Tropical  Agri- 
culture and  Citrus  Experiment  Station,  Biverside,  California. 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 


FOREWORD 

An  attempt  has  been  made  to  present  this  summary  of  the  results 
of  a  survey  of  citrus  orchard  practices  in  southern  California  in  a 
manner  that  would  appeal  to  the  orchardist.  For  the  benefit  of  those 
who  are  interested  in  the  methods  followed,  either  in  the  collection  or 
the  analysis  of  data,  certain  notes  have  been  included  in  smaller  type 
as  an  appendix.  These  notes  will  also  be  of  interest  to  any  who  wish 
to  consider  critically  the  reliability  of  the  conclusions  drawn. 


INTRODUCTION 

Purpose  of  Bulletin 

The  purpose  of  this  bulletin  is  to  show  from  actual  field  records 
the  influence  of  fertilization,  irrigation,  plowing,  climate,  soil,  the 
age  of  trees,  and  costs  upon  the  profitableness  of  citrus  orchards  in 
southern  California. 

IMPORTANCE   AND   GROWTH   OF   CITRUS    INDUSTRY 

The  commercial  citrus  industry  in  California  is  but  little  over 
fifty  years  old.  It  has  developed  rapidly  for  an  agricultural  industry. 
In  1921  California  produced  73  per  cent  of  all  the  oranges  and  88 
per  cent  of  all  the  lemons  consumed  in  the  United  States  and  Canada. 
The  shipments  of  oranges  alone  from  California  averaged  approxi- 
mately 1000  cars  each  week  throughout  the  year. 

During  the  past  ten  years  (1913-14  to  1922-23  inclusive)  ship- 
ments of  oranges  have  increased  about  25  per  cent  while  shipments  of 
lemons  have  more  than  doubled.  Table  1  gives  the  approximate  ship- 
ments in  packed  boxes  for  that  period. 

TABLE  1 

Shipments  of  Packed  Boxes  of  California  Oranges  and  Lemons  for  the 
Season  1913-14  Through  1922-23 
(000  omitted) 
Year1  Oranges        Lemons  Year*  Oranges        Lemons 

1913-14 17,900  1180  1918-19 18,100  4280 

1914-15 15,700  2800  1919-20 15,650  3780 

1915-16 15,100  2900  1920-21 20,200  4700 

1916-17 19,700  3260  1921-22 12,7003  4150 

1917-18 7,8002  2560  1922-23 21,0004  5100« 

1  November  1  to  October  31  *  Crop  reduced  by  cold. 

*  Crop  reduced  by  heat.  *  Partially  estimated. 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  5 

The  average  yield  of  mature  orange  groves  over  a  period  of  years 
is  about  150  packed  boxes  per  acre,  while  that  of  lemons  is  about  175 
boxes.  Wide  variations  from  these  figures  are  occasionally  noted 
for  favorable  seasons  or  on  individual  groves.  A  few  extreme  yields 
of  800  to  1000  packed  boxes  per  acre  are  on  record  for  both  oranges 
and  lemons  but  such  are  not  safe  figures  on  which  to  base  a  judgment 
of  the  industry. 

The  geographic  distribution  of  the  citrus  plantings  has  changed  in 
recent  years.  The  early  plantings  of  citrus  trees  centered  around 
several  locations — notably  San  Gabriel,  Orange,  Riverside,  and  Red- 
lands.  However,  following  the  advent  of  the  navel  orange  in  1873, 
there  was  a  rapid  increase  of  acreage  in  the  interior  districts.  That 
variety  had  first  been  tested  at  Riverside  and  its  early  reputation  was 
that  of  an  interior  fruit.  Later  the  plantings  of  navels  spread  so  that 
it  is  now  the  predominant  orange  throughout  the  Pomona-San  Gabriel 
valleys.  Plantings  of  Valencias — the  summer  orange — gained  im- 
petus some  years  later  and  are  concentrated  to  a  great  extent  in  the 
regions  nearer  the  coast.  Thus  the  average  age  of  orchards  near  the 
coast  is  less  than  that  in  the  interior. 


COLLECTION  OF  DATA 

For  some  years  the  Citrus  Experiment  Station  has  endeavored  to 
bring  together  many  of  the  accumulating  experiences  of  growers  in 
this  new  industry.  Beginning  in  1914  the  writer  collected  statements 
from  individual  growers  showing  their  methods  of  production,  costs 
and  yields.  Much  of  this  material  has  been  of  such  a  heterogeneous 
nature  that  it  could  not  be  satisfactorily  analyzed.  However,  the 
figures  on  cost  of  developing  citrus  groves  are  of  such  nature  that 
they  may  be  reported. 

During  the  winter  of  1922-23  a  more  intensive  survey  was  under- 
taken. Approximately  one  thousand  growers  were  interviewed  and 
specific  information  gathered  from  them  concerning  their  orchard 
practices,  costs  and  yields.  The  only  records  that  were  used  in  sub- 
sequent analysis  were  those  that  covered  a  five-year  operating  period. 

In  this  work  the  writer  had  the  hearty  cooperation  of  the  County 
Agent  and  the  Citrus  Committee  of  the  Farm  Bureaus  in  each  county 
in  which  he  worked.  (Data  were  collected  in  Los  Angeles,  San 
Bernardino,  Riverside,  Orange,  and  Ventura  counties.)  The  usual 
method  of  procedure  was  for  the  County  Agent  to  call  a  meeting  of 
the  Farm  Bureau  Center  in  each  of  the  districts  to  be  surveyed.    The 


6  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

details  of  the  work  were  presented  at  the  meeting.  Then  for  several 
days  following,  interested  growers  made  appointments  for  the  investi- 
gator and  accompanied  him  on  his  survey  calls  upon  the  orchardists 
who  furnished  the  data.  Ordinarily  a  week  was  spent  in  each  locality 
and  a  different  grower  accompanied  the  investigator  each  day. 

This  method  of  gathering  data  naturally  led  to  a  sort  of  selection 
of  groves.  Only  those  were  taken  for  which  a  five-year  record  was 
available,  which  made  it  practically  essential  that  one  owner  should 
have  held  the  property  for  that  length  of  time.  Many  growers  were 
eliminated  because  they  had  kept  no  records  covering  so  long  a  period. 
All  groves  under  eight  years  of  age  were  eliminated ;  and  only  a  small 
number  were  taken  under  fifteen  years.  Many  times  it  was  necessary 
to  leave  out  groves  because  the  owner  held  two  or  more  separate 
orchards  without  having  separate  records  of  their  maintenance. 

Two  things  were  very  evident  in  the  selection  of  groves.  In  the 
first  place  not  over  half  of  the  growers  keep  any  record  of  their 
orcharding  operations  except  as  to  cash  expenses  and  receipts.  And 
in  the  second  place  an  even  smaller  number  attempt  to  analyze  such 
records  as  they  do  keep  with  a  view  either  to  increasing  production 
or  to  decreasing  costs.  For  many  purposes  it  is  more  important  to 
know  how  much  nitrogen  is  applied  to  trees  than  to  know  how  much 
money  is  spent  for  fertilizer.  The  significance  of  such  a  physical 
measure  seems  not  to  have  caught  the  attention  of  the  average  grower. 
Possibly  the  analysis  of  these  survey  data  will  point  to  a  helpful 
method  of  studying  the  orcharding  business. 


FACTOES   UPON  WHICH   INFORMATION   WAS   GATHERED 

There  are  many  variable  factors  in  so  complex  an  industry  as  the 
production  of  citrus  fruits.  This  study  does  not  include  them  all. 
Some  of  the  items  considered  are  the  location  of  groves,  both  in  regard 
to  climate  and  to  soil,  the  variety  and  age  of  the  tree,  and  orchard 
practices  as  to  fertilization  and  irrigation.  Certain  other  items — 
notably  pruning  and  pest  control — have  been  considered  only  inci- 
dentally. Whenever  severe  pruning  had  obviously  affected  the  yields 
of  a  grove  or  whenever  pests  had  not  been  commercially  controlled, 
the  groves  were  discarded  from  the  records.  A  discussion  of  the 
classification  of  the  variable  factors  will  be  found  in  the  appendix. 


Bulletin    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


ANALYSIS  OF  THE  DATA 

In  the  analysis  of  any  production  data  there  are  always  two 
distinct  phases  involved.  The  first  of  these  is  the  relation  between 
changes  in  any  variable  on  the  one  hand  and  physical  production  on 
the  other.  This  relationship  is  always  subject  to  the  law  of  dimin- 
ishing physical  returns,  and  if  the  data  are  complete  enough  they  may 
be  studied  in  the  light  of  this  law.  The  practice  which  results  in  the 
greatest  yield  per  acre  may  be  called  the  ' '  most  productive  practice. ' n 

The  second  phase  of  the  analysis  is  concerned  with  the  effect 
which  changes  in  any  variable  have  on  the  profitableness  of  the  enter- 
prise. When  viewed  from  this  angle,  the  data  are  to  be  interpreted 
in  accordance  with  their  approach  to  the  "  limit  of  profitable  culti- 
vation." The  practice  which  results  in  the  greatest  net  returns  on 
the  investment  may  be  called  the  "most  profitable  practice." 

The  most  productive  practice  and  the  most  profitable  practice  are 
such  totally  different  concepts  that  they  require  separate  consider- 
ation. In  this  discussion  physical  returns  will  be  considered  first  and 
then  some  of  the  more  salient  factors  concerned  with  the  "limit  of 
profitable  cultivation"  will  be  presented. 


RELATION  OF  CITRUS  YIELDS  TO  ENVIRONMENT  AND 
ORCHARD  PRACTICES 

CLIMATIC    ZONES    AND    THEIR    EFFECTS    ON    ORCHARD    PRACTICES 

AND  YIELDS 

The  citrus  belt  of  southern  California  comprises  districts  which 
possess  markedly  different  climates.  The  climate  at  Santa  Ana  is 
quite  different  from  that  at  Riverside,  particularly  during  the  summer 
months.  Santa  Ana  is  typically  under  a  coastal  influence,  while  River- 
side approaches  the  arid  interior.     The  Pomona  Valley  lies  midway 


1  "  It  is  well  known  to  practical  men  that  a  niggardly  application  of  labor  and 
capital  to  a  piece  of  land  in  the  cultivation  of  any  crop  is  little  better  than  wasted, 
because  it  will  produce  so  little  in  proportion  to  itself;  whereas  a  more  generous 
application  will  yield  a  crop  not  only  larger,  but  larger  in  proportion  to  the  amount 
of  labor  and  capital  employed.  Up  to  this  point  the  land  is  said  to  yield  increas- 
ing returns  to  the  labor  and  capital  employed  in  its  cultivation.  But  if  the 
amount  of  these  factors  used  in  cultivating  a  given  piece  of  land  is  still  further 
increased,  a  point  will  eventually  be  reached  where  the  product  will  no  longer 
increase  as  fast  as  these  factors  are  increased.  Beyond  this  point  the  land  is  said 
to  yield  diminishing  returns  to  the  labor  and  capital  employed.  Though  larger 
applications  of  labor  and  capital  may  continue  to  produce  larger  crops,  the  crops 
will  not  be  so  large  in  proportion  to  the  labor  and  capital. "  T.N.  Carver,  The 
Distribution  of  Wealth,  page  56.  The  application  of  the  law  of  diminishing 
returns  to  the  present  data  is  discussed  further  on  page  40. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

between  the  two  in  the  amount  of  coastal  influence  which  it  receives. 
In  this  study  the  citrus  belt  has  been  divided  into  three  climatic  zones 
as  represented  by  the  above  illustrations. 

The  division  of  the  southern  California  citrus  belt  into  three 
climatic  zones  is  somewhat  arbitrary.  The  main  criterion  used  in  the 
division  was  the  mean  maximum  temperature  during  the  summer. 
On  this  basis  the  coastal  zone  includes  most  of  Orange  County  and 
the  Whittier  and  Pasadena  sections  of  Los  Angeles  County;  the 
intermediate  zone  includes  the  San  Gabriel  and  Pomona  valleys ;  the 
interior  zone  includes  the  San  Fernando,  Fillmore,  Redlands,  High- 
lands and  Riverside  districts. 

The  Government  Weather  Bureau  reports  show  consistent  differ- 
ences in  mean  monthly  maximum  temperatures  during  the  summer 
between  these  three  zones.  The  month  of  August  has  been  used  as 
an  index  of  these  differences,  and  all  the  stations  within  each  zone 
have  been  averaged  over  a  five-year  period.  The  figures  in  table  2 
result. 

TABLE  2 

Average  Mean-Monthly-Maximum  August  Temperatures,  for  Each  Climatic 

Zone,  1917-1921 

Coastal   87°  F 

Intermediate    91°  F 

Interior     94°  F 

(Tulare  County  98°  F) 

(Not  included  in  this  survey.) 

The  average  annual  rainfall  for  seven  stations  in  each  zone  is 
given  in  table  3.  These  averages  are  based  on  the  records  of  the 
United  States  Weather  Bureau  and  cover  from  25  to  40  years  in 
individual  cases. 

TABLE  3 
Average  Annual  Eainfall  in  Climatic  Zones 

Coastal    15.6  inches 

Intermediate    19.6  inches 

Interior  16.9  inches 

Other  weather  factors  of  recognized  importance  to  crops  are 
relative  humidity  and  wind.  Complete  data  on  these  points  are 
lacking,  but  the  effect  of  the  three  factors  of  temperature,  relative 
humidity,  and  wind  may  be  estimated  by  the  relative  rate  of  evapora- 
tion of  water  from  the  surface  of  porous  porcelain  spheres.  This 
rate  in  the  interior  district  is  approximately  double  that  of  the  coastal 
district. 


Bulletin   374] 


ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


The  range  of  soils  is  roughly  similar  in  the  three  districts  although 
there  is  a  slightly  lower  proportion  of  heavy  soil  in  the  intermediate 
zone.  The  classification  of  soils  that  was  finally  adopted  for  this 
study  includes  loams  and  clay  loams  as  "heavy";  fine  sandy  loams 
as  "medium";  sandy  loams  and  gravelly  sandy  loams  as  "light." 

Certain  data  concerning  the  average  yields  and  practices  in  the 
three  zones  are  given  in  table  4. 

TABLE  4 
Kelation  Between  Climatic  Zones,  Average  Orchard  Practices,  and  Yields 


Age  of 
trees 
(years) 

Yield 
per  acre 

Fertilization 
per  acre 

Irrigation 
per  acre 

Zone 

Oranges 
(pounds) 

Lemons 
(pounds) 

Nitrogen 
applied 
(pounds) 

Manure 
applied 
(pounds) 

Water 
applied 

(acre 
inches) 

Fre- 
quency 

(days) 

Coastal 

21 
25 
27 

19,500 
18,500 
15,400 

24,700 
24,000 
18,900 

123 
160 
122 

9.6 

7.8 
6.3 

20.2 
24.9 
28.6 

33 

Intermediate 

Interior 

25 
30 

It  is  probable  that  part  of  the  differences  in  yield  among  these 
different  districts  is  due  to  climatic  conditions.  It  is  equally  probable 
that  part  of  the  differences  may  be  due  to  differences  in  soil,  age  of 
trees,  and  orchard  practices. 

Lemons  produce  more  fruit  per  acre  than  oranges  in  each  of  the 
districts.  The  excess  is  less  in  the  interior  than  elsewhere,  indicating 
that  lemon  production  is  at  a  relative  disadvantage  as  compared  with 
orange  production  in  that  zone.  The  excess  of  lemon  production  per 
acre  over  orange  production  for  the  several  zones  is : 

Coastal    40  per  cent,  or  5200  pounds 

Intermediate    42  per  cent,  or  5500  pounds 

Interior  23  per  cent,  or  3500  pounds 


Effect  of  Abnormal  Weather  Conditions  on  Yields 

The  records  given  in  table  1  show  the  widespread  effect  of  unfavor- 
able weather  conditions  in  two  separate  years.  In  June,  1917,  there 
occurred  a  few  unusually  hot  days  that  resulted  in  a  heavy  drop  of 
young  fruit,  while  in  February,  1922,  at  least  50  per  cent  of  the 
orange  crop  was  frozen  on  the  trees.  Table  5  presents  the  fluctua- 
tions in  shipments  from  one  of  the  oldest  citrus  districts  in  the  state. 


2  For  a  further  discussion  of  citrus  orchard  soils  see  ' '  Fertilizer  Experiments 
with  Citrus  Trees,"  E.  S.  Vaile.  Calif.  Agr.  Exp.  Sta.,  Bull.  345,  pp.  508-511. 
June,  1922. 


10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  bearing  acreage  in  this  district  continued  to  increase  until  1911 
or  1912.  Since  that  time  there  has  been  a  slight  reduction  in  total 
acreage. 

TABLE  5 

Shipment  of  Citrus  Fruits  from  One  of  the  Oldest  Citrus  Districts  for  the 

Seasons  1890-91  to  1921-22  Inclusive 

Packed  boxes   (000  omitted) 


Year 

Boxes 

Year 

Boxes 

1890-91 

415 

400 

715 

5301 

800 

585 

715 

1340 

7151 

1310 

1580 

10901 

1410 

1710 

1730 

15201 

1906-07  

1630 

1891-92 

1907-08 

1480 

1892-93 

1908-09  

1740 

1893-94 

1909-10  

11202 

1894-95 

1910-11 

1640 

1895-96 

1911-12  

8301 

1896-97 

1912-13 

2501 

1897-98 

1913-14 

1060 

1898-99 

1914-15 

910 

1899-00 

1915-16 

900 

1900-01 

1916-17 

1050 

1901-02 

1917-18 

4402 

1902-03 

1918-19 

1050 

1903-04 

1919-20  

1080 

1904-05 

1920-21 

1480 

1905-06 

1921-22  . 

12501 

Cold  affected  yields. 


Heat  affected  yields. 


EFFECT  OF  SOILS  ON  YIELDS 

Soils  may  be  grouped  in  several  different  ways.  In  this  study  the 
classifications  of  the  U.  S.  Bureau  of  Soils  have  been  followed.  When 
divided  according  to  geological  origin  and  formation,  the  citrus  soils 
mainly  fall  into  four  series,  namely,  Hanford,  Yolo,  Ramona,  and 
Placentia.  All  of  these  are  alluvial  soils.  The  Hanford  and  Yolo 
series  are  relatively  recent,  while  the  Ramona  and  Placentia  are  much 
older.  Yolo  soils  are  derived  from  shales  and  other  sedimentary  rocks, 
while  the  other  three  series  are  derived  mainly  from  granitic  rocks. 
Each  of  these  series  is  represented  in  all  phyiscal  grades  from  sandy 
loam  to  clay  loam. 

There  is  no  apparent  difference  in  average  yield  per  acre  of  citrus 
on  Yolo,  Hanford,  or  Ramona  soils  of  the  same  physical  grade. 
Placentia  soils,  which  are  often  underlain  by  hardpan,  and  which 
absorb  irrigation  water  very  slowly,  are  less  productive  of  citrus  than 
any  of  the  others. 


Bulletin   374] 


ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


11 


The  physical  make-up  of  the  soils  is  of  some  importance  in  its 
effect  on  citrus  yields.  The  groves  planted  on  medium-textured  soil 
(fine  sandy  loam)  gave  the  highest  average  yield.  About  5  per  cent 
of  the  groves  are  planted  on  very  light  soil  (sand)  and  these  groves 
produced  30  per  cent  less  than  those  on  medium-textured  soil.  From 
5  per  cent  to  10  per  cent  of  the  groves  were  planted  on  very  heavy 
soil  (clay  loam)  and  these  produced  10  per  cent  less  than  those  on 
medium-textured  soil.  Table  6  gives  the  actual  average  yields  for  the 
several  types.  Figure  1  is  a  graphic  presentation  of  the  yields  in 
the  intermediate  zone. 

TABLE  6 
Average  Yields  of  Oranges  on  Different  Soil  Types,  in  Pounds  per  Acre 


Type 


Yield  in  pounds  per  acre  by 
climatic  zones 


Coastal 


Intermediate 


Interior 


Sand — very  light 

Gravelly  sandy  loam  and  sandy  loam — light 

Fine  sandy  loam — medium 

Loam — heavy 

Clay  loam — very  heavy 


14,400 
20,100 
21,200 
19,600 
19,000 


13,700 
19,200 
20,100 
18,700 
17,900 


11,400 
16,000 
16,700 
15,500 
14,900 


The  differences  in  yields  between  the  middle  ranges  of  soils  (light, 
medium,  and  heavy)  are  so  slight  that  they  may  be  almost  disregarded. 
Comparatively  few  groves  have  been  planted  on  the  extreme  soil  types 
where  a  real  reduction  of  yields  is  experienced.  In  the  following 
study  of  effect  of  orchard  practices  on  yields,  the  groves  planted  on 
sand  and  those  planted  on  clay  loam  have  been  excluded. 


AGE  OF  TREES  AND  YIELDS 

There  appears  to  be  a  definite  relationship  between  the  age  of 
orange  trees  and  their  production.  A  regular  and  persistent  increase 
in  average  production  occurs  until  the  groves  are  at  least  thirty-five 
years  old.  The  small  number  of  groves  listed  that  are  forty  years  old 
indicate  a  still  further  increase,  but  the  number  of  examples  is  too 
small  to  warrant  safe  conclusions.  Possibly  this  apparently  regular 
increase  is  due  to  the  method  of  grove  selection;  certain  it  is  that 
many  groves  have  been  abandoned  in  the  past  as  unprofitable  before 
reaching  the  age  of  thirty-five  years.  However,  the  data  indicate  that 
well  located  groves  may  be  expected  to  continue  to  yield  satisfactorily 
at  least  to  thirty-five  or  forty  years  of  age  if  given  reasonable  care. 
The  actual  yields  for  the  different  age  groups  are  given  in  table  7. 


12 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Age  of  Trees 

TABLE  7 
and  Production 

r  of  Oranges 

Age 

Yield  in  pounds  per  acre 

Coastal 

Intermediate 

Interior       LSj 

10 

13,100 
18,700 
19,300 
20,100 
20,500 
20,900 

9,600 
17,700 
18,300 
19,050 
19,450 
19,800 

8,900 

15 

14,700 
15,200 
15,850 
16,200 
16,500 

20 

25             

30 

35 

If  the  35-year-old  group  is  considered  as  being  in  full  bearing, 
the  younger  groves  may  be  represented  by  percentages  of  a  full 
yield  as  given  in  table  8.  The  data  are  graphically  presented  in 
figure  2. 

So/7     Types  and    Yields 

Co  os  fa/    D/s  trie  t 
Yield  in 
Pounds  Per  Acre. 

ZZ500 
20,000 

/Zsoo 

/ 5,000 
/2,50O  - 

/0,00a  - 


So/'/ 
Types 


^ 

■^1 

5 

s 

H^                       _S 

r 

^                       ^ 

^          $ 

Fig. 

1.     Illustrating  table 

I 

r 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  13 

TABLE   8 

Relative  Yields  of  Orchards  by  Ages  when  the  35-Year-Old  Groves  are 
Considered  Full  Bearing 

Age  Relative  yield 

35 100 

30 97 

25 94 

20 92 

15 89 

10 60 

Age  of  Trees  and  /Relative  Yields. 


fte/af/ve 
Yields. 


/OO  r 
90 
80 
70    - 
60 
SO 


fO  /S  20         25         JO         35 

Age  of  Trees  in  Years 

Fig.  2.     Illustrating  table  8. 


14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  rate  of  increase  in  yields  is  almost  identical  in  each  of  the 
three  climatic  zones  except  that  the  10-year-old  groves  produce  rela- 
tively more  near  the  coast  than  elsewhere.  The  three  percentages 
stated  separately  for  that  age  are: 

Relative  yields 
Coastal        Intermediate     Interior 

10-year  group 67  52  58 

Lemon  trees  increase  in  yields  at  essentially  the  same  rate  as  do 
orange  trees. 

The  10-year-old  group  has  been  eliminated  in  the  subsequent 
studies  of  the  effect  of  orchard  practices  on  yields. 


FERTILIZATION  AND  ITS  EFFECT  ON  YIELDS 

The  successful  orchardists  of  the  citrus  industry  have  realized  for 
some  time  that  the  supply  of  nitrogen  available  to  the  trees  is  one 
of  the  dominant  factors  of  fruit  production.  The  orchard  trials  at 
the  Citrus  Experiment  Station  (Rubidoux  plots)  have  strongly  em- 
phasized this  fact  during  the  past  ten  or  more  years.  The  same 
general  conclusion  was  reached  in  the  Citrus  Experiment  Station 
trials  at  Arlington  and  the  cooperative  trials  that  have  been  conducted 
in  various  parts  of  the  state.  Comparatively  little  evidence  has  ex- 
isted, however,  as  to  the  exact  amount  of  nitrogen  that  should  be 
used.3  The  present  study  gives  a  general  basis  for  an  answer  to  this 
question.  In  this  study  the  total  amount  of  nitrogen  carried  in  con- 
centrated commercial  fertilizers  is  considered  together  with  an  esti- 
mate of  the  amount  carried  in  all  applications  of  bulky  organic 
manures. 

Yields  as  Affected  by  Variations  in  Nitrogen  Applications 

The  data  gathered  in  this  survey  show  a  regular  increase  in  aver- 
age yields  until  about  350  pounds  of  actual  nitrogen  are  applied  per 
acre.  With  larger  applications  there  is  an  apparent  decrease  in 
yields.  The  observed  yields  with  varying  fertilizer  applications  are 
given  as  percentages  of  the  average  yields  in  table  9.  This  table 
applies  equally  to  the  groves  from  each  of  the  climatic  zones.  The 
method  of  combining  the  zones  is  fully  explained  on  page  39  of  the 
appendix.    The  data  are  also  presented  in  figure  3. 

s  Fertilizer  experiments  with  Citrus  Trees,  by  R.  S.  Vaile.  Calif.  Agr.  Exp. 
Sta.  Bull.  345,  June,  1922.     See  especially  page  508. 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


15 


TABLE  9 
Nitrogen  Applied  and  Eelative  Orange  Yields 

Nitrogen 

in  pounds  Relative 

per  acre  yields 

501 76 

100 94 

150 106  • 

200 114 

250 128 

300 135 

350 143 

400 130 

1  Given  as  the  center  of  the  class,  the  limits  of  which  are  26  to  75,  etc. 

These  relative  yields  may  be  converted  back  into  approximate,  real 
yields  by  using  the  average  yield  for  district  and  variety  as  100. 
Table  10  gives  the  yields  that  should  be  expected  under  average  con- 
ditions with  the  use  of  the  amount  of  nitrogen  indicated. 


Nitrogen    Applied      And    Relative    Yields 

With    Curve     ol    D/m/n/sri/nj     Returns 


/oo 


/so 


2CO 


250 


300 


350 


A// 


fro  gen  Applied  //?     Pounds   £br  Acre 


Fig.  3.  The  circles  represent  average  yields  for  each  fertilizer  class,  as  given 
in  table  9.  The  curve  represents  the  yield  to  be  expected  in  response  to  the  law 
of  diminishing  returns  from  additional  applications  of  nitrogen,  as  given  in 
table  27. 


16 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  10 
Nitrogen  Applied  and  Yields 


Nitrogen  in 

Yield  in  thousands  of  pounds  per  acre 

pounds  per 
acre 

Coastal 

Intermediate 

Interior 

Oranges 

Lemons 

Oranges 

Lemons 

Oranges 

Lemons 

50 
100 
150 
200 
250 
300 
350 
400 

14.8 
18.3 
20.7 
22.2 
25.0 
26.3 
27.9 
25.4 

18.7 
23.2 
26.2 
28.2 
31.6 
33.4 
35.3 
32.0 

13.6 
16.5 
18.5 
20.0 
22.4 
23.6 
25.0 
22.8 

17.2 
21.2 
24.0 

25.8 
28.9 
30.5 
32.3 

29.4 

11.7 
14.4 
16.3 
17.6 
19.7 
20.4 
22.0 
20.0 

14.3 
17.8 
20.0 
21.5 
24.2 
25.6 
27.1 
24.6 

Note: — These  figures  are  based  on  an  average  of 
fruit  shipped. 


pounds  of  fruit  picked  to  each  packed  box  of 


All  groves  that  received  as  much  as  350  pounds  of  nitrogen  per 
acre  annually  were  badly  mottled,  although  most  of  them  were  still 
producing  better  than  average  crops.  A  considerable  amount  of 
mottle-leaf  was  observed  on  the  groves  receiving  250  to  350  pounds 
of  nitrogen.  This  bears  out  the  observations  made  on  the  Arlington 
experimental  plots  in  1915-16,  when  a  heavy  application  of  alfalfa 
hay,  carrying  nearly  400  pounds  of  nitrogen  to  the  acre,  was  followed 
by  a  severe  case  of  mottling  which  did  not  disappear  for  several  years. 
Dr.  I.  G.  McBeth  has  also  reported  that  extreme  mottling  is  often 
associated  with  high  nitrate  content  in  the  soil.4  It  seems  probable, 
because  of  this  association,  that  the  most  productive  use  of  nitrogen 
for  citrus  orchards  is  reached  somewhere  between  300  and  350  pounds 
per  acre  annually.    The  most  profitable  use  will  be  discussed  later. 

An  interesting  trial  of  the  effect  of  varying  amounts  of  nitrogen 
on  yield  has  been  conducted  on  a  mature  orange  grove  at  Ontario  by 
the  Chaffey  Junior  College  of  Agriculture.5  Three  sections  of  orchard 
have  been  given  100  pounds,  200  pounds,  and  300  pounds  of  nitro- 
gen respectively.  In  each  case  one-half  the  nitrogen  has  come  from 
manure  and  the  other  half  from  a  quick-acting  concentrate.  For 
several  years  prior  to  1921  the  entire  grove  had  been  lightly  but 
uniformly  fertilized.  The  yield  in  1922-23,  after  the  second  appli- 
cation of  the  varying  amounts,  was  as  follows: 

*  ( l  Soil  nitrogen  and  nutrition  of  citrus  plants, ft  by  I.  Gr.  McBeth.  Jour,  of 
Agr.  Kesearch,  vol.  9,  no.  7,  pp.   248-251.     May,  1917. 

s  The  data  of  the  Chaffey  Junior  College  of  Agriculture  experiment  were  fur- 
nished through  the  courtesy  of  Mr.  C.  A.  Booth. 


Bulletin    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  17 

Pounds  nitrogen  Pounds  of  fruit 

per  acre  per  acre 

100 14,300 

200 18,300 

300 18,700 

Three  hundred  pounds  of  nitrogen  per  acre  gave  the  most  pro- 
ductive use,  but  obviously  two  hundred  pounds  represented  the  most 
profitable  use  in  this  case. 

Of  the  growers  interviewed,  30  per  cent  are  using  only  100  pounds 
of  nitrogen  per  acre  while  75  per  cent  are  using  less  than  225  pounds. 
Approximately  half  of,  the  nitrogen  applied  has  been  carried  in 
bulky  organic  manure.  It  seems  probable  that  citrus  production  may 
be  more  surely  and  rapidly  increased  by  increasing  the  applications 
of  fertilizer  than  in  any  other  way. 

To  apply  350  pounds  of  nitrogen,  half  of  which  is  carried  in  bulky 
organic  manure,  one  must  use  20  tons  of  stable  manure  together  with 
1500  pounds  of  dried  blood  or  nitrate  of  lime  per  acre,  or  other 
materials  that  will  supply  equivalent  amounts  of  nitrogen.  These 
are  large  quantities  and  will  cost  not  less  than  $150  per  acre  for 
fertilizer  alone.  Only  a  few  growers  use  such  amounts  consistently 
year  after  year. 

Five  tons  of  alfalfa  hay,  or  eight  tons  of  either  Lima  bean  straw, 
first  grade  chicken  manure,  or  clean  sheep  manure  might  be  substi- 
tuted for  the  twenty  tons  of  stable  manure.  One  thousand  pounds 
of  sulfate  of  ammonia  or  3000  pounds  of  cottonseed  meal  carry 
approximately  the  same  amounts  of  nitrogen  as  1500  pounds  of  blood. 
The  choice  between  the  different  materials  within  the  two  groups  is 
probably  to  be  made  entirely  on  the  basis  of  relative  costs,  for  the 
difference  in  their  fertilizing  values  is  apparently  proportional  to  the 
amount  of  nitrogen  which  they  carry. 

In  the  discussion  above,  the  most  productive  use  of  nitrogen  has 
been  considered.  Later  certain  of  the  factors  which  determine  the 
most  profitable  use  will  be  presented. 

The  Use  of  Balky  Organic  Manures,  and  Yields 

The  Citrus  Experiment  Station  field  trials  (Rubidoux  plots)  have 
emphasized  the  fact  that  concentrated  nitrogenous  fertilizers  used 
persistently  without  bulky  organic  material  will  not  permanently 
maintain  healthy  citrus  trees  under  the  conditions  which  prevail  at 
Riverside.  Casual  field  observations  the  state  over  bear  out  this 
conclusion.  Groves  are  occasionally  successfully  managed  for  as  long 
as  ten  years  with  almost  all  the  fertilizer  applied  in  concentrated 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

form.  The  day  of  reckoning  is,  however,  sure  to  come.  In  the 
writer's  observations  there  are  a  score  or  more  of  groves  that  at  one 
time  were  looked  upon  as  "show  places,"  but  that  have  since  de- 
teriorated rapidly  through  failure  to  recognize  this  principle. 

The  survey  data  have  been  tabulated  in  two  ways  in  order  to 
measure  the  effect  of  applications  of  organic  matter  on  yields.  First, 
all  of  the  groves  were  grouped  into  three  classes  according  to  the 
proportion  of  the  total  nitrogen  used  which  was  carried  in  bulky 
organic  manures.     The  class  divisions  were: 

(1)  Less  than  40  per  cent  of  nitrogen  carried  in  bulky  manure ; 

(2)  40  to  60  per  cent  of  nitrogen  carried  in  bulky  manure; 

(3)  Over  60  per  cent  of  nitrogen  carried  in  bulky  manure. 
The  data  show  that   the  largest   average  yields  resulted  when   the 
largest  proportion  of  the  nitrogen  was  carried  in  bulky  manure.    Call- 
ing the  average  yield  in  this  class  100,  table  11  may  be  compiled. 

TABLE  11 

Relation  of  Nitrogen  in  Bulky  Manures  to  Yields 

Nitrogen  in  Relative 

bulky  manures  yields 

Over  60  per  cent 100 

60-40  per  cent 93 

Under  40  per  cent 89 

The  use  of  nitrogen  so  completely  dominates  yields  that  a  direct 
correlation  between  manure  used  and  yields  is  not  high.  However, 
as  a  second  method  of  grouping,  table  12  indicates  that  more  manure 
does  bring  more  fruit.  The  highest  average  class  yield  is  arbitrarily 
taken  as  100. 

TABLE  12 
Eelation  of  Applications  of  Manure  to  Yield 

Manure  applied  Relative 

tons  per  acre  yields 

0—4.9 86 

5—9.9 92 

10—14.9 95 

15—19.9 100 

20  and  over '. 99 

Throughout  the  study  of  organic  manure,  the  high-grade  materials 
such  as  alfalfa  hay  have  been  converted  into  equivalent  amounts  of 
ordinary  manure  by  multiplying  by  the  ratio  of  organic  matter  which 
they  contain. 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


19 


Effect  of  Cover  Crops  on  Fertilizer  Requirements 

During  the  past  ten  years  the  use  of  winter  green-manure  crops  in 
citrus  groves  has  materially  increased. 

In  the  winter  of  1914-15  it  was  estimated  that  less  than  40,000 
acres  of  citrus  orchards  were  planted  to  green-manure  crops  of  any 
kind.  Nearly  half  of  this  acreage  was  planted  to  cereal  crops.  In 
1915-16  approximately  67,000  acres  were  planted  to  leguminous, 
green-manure  crops,  and  in  1916-17  approximately  100,000  acres 
were  so  planted.  Since  1917  there  have  been  fluctuations  in  planting 
with  an  apparent  upward  trend. 

In  an  attempt  to  measure  the  immediate  effect  of  winter  green- 
manuring  on  citrus  production,  the  grove  records  were  first  sorted 
according  to  the  amount  of  nitrogen  applied  and  then  each  class  was 
subdivided  into  cover-cropped  and  clean-cultivated  groups.  The  aver- 
age yields  were  practically  identical  for  the  cover-cropped  and  clean- 
cultivated  groves  in  each  nitrogen  class.  Apparently  the  use  of  cover 
crops  does  not  diminish  the  necessity  for  nitrogen  applications. 

The  records  were  also  divided  into  classes  according  to  the  amount 
of  bulky  manure  used  and  subdivided  into  winter-cover-cropped  and 
clean-cultivated  groups.  In  every  case  the  cover-cropped  group  gave 
a  higher  average  yield  than  the  clean-cultivated  group.  The  difference 
was  greatest  where  the  smallest  amount  of  manure  was  used,  and 
became  regularly  less  as  the  requirements  for  organic  matter  were 
better  supplied  from  outside  sources.  Table  13  gives  the  comparative 
yields  when  the  clean-cultivated  groves  in  each  manure  class  are 
counted  as  100.  The  per  cent  above  100  in  the  cover-cropped  group 
is  the  increase  to  be  expected  from  green-manuring,  when  definite 
amounts  of  manure  are  used. 

TABLE  13 

Effect  of  Cover  Crop  on  Average  Yield  of  Groves  Treated  with  Bulky 

Manure 


Manure  applied  tons  per  acre 

Relative  yields 

Without  cover  crop 

With  cover  crop 

0—  4.9 

100 
100 
100 
100 

119 

5—  9.9 

110 

10—14.9 

108 

15—19.9 

106 

20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

When  the  cover-cropped  groves  are  compared  with  the  clean- 
cultivated  groves  that  received  five  tons  more  of  manure  each  year  the 
average  yields  are  practically  identical  (100:101).  It  seems  prob- 
able, therefore,  that  the  persistent  use  of  green-manure  crops  will 
relieve  the  growers  of  the  necessity  of  applying  such  liberal  applica- 
tions of  manure.  It  does  not  seem  probable,  however,  that  mature 
groves  can  be  maintained  in  the  best  condition  without  some  applica- 
tions of  manure,  even  when  winter  cover  crops  are  grown. 

Analysis  made  by  W.  P.  Kelley  indicates  that,  on  the  Rubidoux 
plots  of  the  Citrus  Experiment  Station,  a  winter  green-manure  crop 
and  about  ten  tons  of  manure  per  acre  annually  have  just  kept  the 
organic  content  of  the  soil  equal  to  that  of  virgin  soil.  The  plots  so 
treated  have  produced  more  fruit  than  plots  which  have  received 
thirteen  tons  of  manure  without  the  winter  cover-crop. 

The  experience  of  the  Experiment  Station  on  its  Rubidoux  plots 
where  winter  legumes  have  been  successfully  grown  for  sixteen  con- 
secutive seasons  indicates  that  a  rotation  of  varieties  is  desirable. 
Common  vetch,  Melilotus,  purple  vetch,  and  horse  beans  have  been 
used  at  various  times.  No  single  crop  has  been  used  for  more  than 
three  consecutive  years  and  as  a  result  the  tonnage  of  green  manure 
produced  has  always  been  heavy.  Such  rotation  avoids  some  of  the 
cumulative  injury  from  aphis,  mildew,  and  weeds  because  of  the 
different  habits  and  susceptibility  of  the  several  crops. 

The  number  of  groves  recorded  in  the  survey  in  which  summer 
cover  crops  have  been  grown  for  a  five  year  period  is  small.  Such 
groves  received  large  amounts  of  irrigation  water,  and  their  average 
yields  were  not  so  high  as  those  of  clean-cultivated  groves  receiving 
the  same  amount  of  water.  In  several  instances  the  observation 
was  made  that  the  use  of  summer  cover  crops  for  three  or  more  con- 
secutive years  resulted  in  very  poor  tree  conditions.  It  may  be  pos- 
sible to  grow  summer  cover  crops  in  citrus  orchards  to  advantage, 
but  methods  of  general  applicability  do  not  seem  to  have  been  worked 
out.  i1    '     • ! 

Of  the  1000  groves  listed,  only  two  were  found  on  which  neither 
manure  nor  cover  crop  had  been  used  for  as  long  as  five  years. 
Although  there  is  a  general  recognition  of  the  necessity  of  maintaining 
the  organic  content  of  the  soil,  at  least  75  per  cent  of  the  orchards 
could  be  expected  to  yield  somewhat  larger  crops  if  more  organic 
material  were  used.  Very  substantial  improvement  to  25  per  cent 
of  the  groves  should  result  from  increased  application  of  manure. 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  21 


IRRIGATION  AND  ITS  EFFECT  ON  YIELDS 

Climate  and  soils  each  have  an  important  effect  on  irrigation 
practice  in  citrus  groves.  The  survey  data  indicate  that  less  water  is 
used  in  general  near  the  coast  than  in  the  interior,  while  the  inter- 
vals between  irrigations  are  longer  in  the  cooler  coastal  districts. 
Climate,  therefore,  is  one  important  variable. 

Different  soils  require  different  irrigation,  even  under  the  same 
climatic  conditions.  The  amount  of  water  that  can  be  stored  for 
use  in  a  light  soil  is  much  less  than  in  a  heavy  soil  and  it  would, 
therefore,  naturally  be  used  up  sooner.  This  condition  calls  for  more 
frequent  irrigations  on  light  soils  than  on  heavy  ones  where  the 
climatic  conditions  are  the  same. 

There  is  no  evidence,  on  the  other  hand,  to  show  that  a  given  crop 
will  transpire  more  water  when  grown  on  one  soil  than  when  grown 
on  another,  unless  the  growth  is  more  vigorous  in  one  case  than  in 
the  other.  With  minor  exceptions  citrus  trees  appear  to  grow  equally 
vigorously  on  the  various  grades  of  soil  in  southern  California  and 
therefore  they  should  use  about  the  same  total  amount  of  water, 
regardless  of  soil.  The  survey  data  bear  out  this  reasoning.  There 
is  no  material  difference  in  the  amount  of  water  commonly  applied 
on  different  soils,  and  the  average  yields  with  any  given  amount  of 
water  are  approximately  the  same  on  the  various  soils. 

Amount  of  Water  to  Apply 

To  compare  the  effect  of  increasing  the  amount  of  water  used,  the 
records  from  each  climatic  zone  were  divided  into  five  groups  or 
classes  according  as  the  total  amount  of  irrigation  water  used  was 
light,  usual,  moderate,  heavy,  or  very  heavy.  Different  limits  were 
set  for  these  several  classifications  because  the  range  of  treatments 
was  different  in  the  several  climatic  zones.  The  most  common  usage 
was  made  the  central  class  for  each  zone  and  the  maximum  range  of 
usage  was  equally  divided  into  two  classes  that  used  more  and  two 
classes  that  used  less  than  the  usual  amount.  The  class  divisions  are 
indicated  in  table  14.  The  class  designated  as  " usual' '  represents 
the  amount  most  commonly  used. 

The  average  yields  for  orchards  in  these  several  classes  are  given 
in  table  15. 

In  the  coastal  zone  a  little  less  than  the  amount  of  water  now 
commonly  used  gives  the  highest  yield.  There  is  considerable  evidence 
of  injury  from  over-irrigation  throughout  this  zone,  particularly  on 


22 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


heavy  soil.  This  is  in  general  agreement  with  the  finidngs  of  E.  E. 
Thomas  of  the  Citrus  Experiment  Station  in  his  study  of  citrus  grove 
irrigation  in  the  vicinity  of  Whittier.6 

In  each  of  the  other  zones  the  average  yields  increase  with  the  use 
of  more  water  until  nearly  50  per  cent  more  than  the  usual  amount 
is  applied.  Even  in  these  zones,  however,  the  injurious  effects  of  over- 
irrigation  are  noted  on  certain  groves  that  are  very  heavily  irrigated. 


TABLE  14 

Designation  of  Classes  in  Irrigation 


Class 


Acre  inches  applied  per  season 


Coastal 


Intermediate 


Interior 


Light 

Moderate.... 

Usual 

Heavy 

Very  heavy 


Under  14 
14—17.9 
18—21.9 
22—25.9 
26 — and  over 


Under  16  .5 
16.5—21.4 
21.5—26.4 
26.5—31.4 
31 .5 — and  over 


Under  19 
19—24.5 
25—30.9 
31—36.9 
37 — and  over 


TABLE  15 

Yields  and  Amount  of  Irrigation  Water 


Class 


Light 

Moderate.... 

Usual 

Heavy 

Very  heavy 


Thousands  of  pounds  per  acre 


Coastal 

Intermediate 

Interior 

Oranges 

Lemons 

Oranges 

Lemons 

Oranges 

Lemons 

18.9 

23.9 

17.5 

22.7 

12.3 

15.1 

20.5 

26.0 

18.1 

23.4 

14.9 

18.3 

20.1 

25.6 

18.5 

24.0 

15.5 

19.0 

19.9 

25.5 

20.6 

26.8 

19.1 

23.4 

19.5 

24.7 

22.0 

28.0 

17.7 

21.7 

Irrigation  water  in  the  intermediate  and  interior  zones  is  so  scarce 
and  so  expensive  that  the  most  profitable  use  may  be  reached  some- 
time before  the  most  productive  use.  This  question  can  only  be 
decided  when  the  cost  of  developing  additional  water  for  a  given 
locality  is  known. 


e  Studies  on  the  irrigation  of  citrus  groves,  by  E.  E.  Thomas.     Calif.  Agr.  Exp. 
Sta.  Bull.  341.     March,  1922. 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  23 

Frequency  at  Which  to  Irrigate 

The  records  indicate  that  longer  intervals  between  irrigations  are 
more  desirable  near  the  coast  than  in  the  interior.  The  highest  aver- 
age yields  for  the  groups  divided  by  intervals  occurred  in  the  fol- 
lowing classes: 

Coastal,  34-41  days  or  approximately  5  weeks; 
Intermediate,  25-33  days  or  approximately  4  weeks; 
Interior,  18-24  days  or  approximately  3  weeks. 

The  most  common  irrigation  interval  in  the  intermediate  district 
is  apparently  well  adapted  to  the  needs  of  the  trees  (25  days).  The 
present  common  usage  in  the  interior  allows  too  long  an  interval 
between  irrigations  (30  days)  for  maximum  average  yields,  while 
near  the  coast  the  common  interval  (33  days)  is  too  short  for  most 
productive  use. 

These  findings  in  regard  to  intervals  between  irrigation  are  borne 
out  by  soil  moisture  studies  in  the  several  districts. 

PLOWING  AND  ITS  EFFECTS  ON  YIELDS 

Just  about  half  of  the  groves  recorded  were  plowed  each  spring 
during  the  five-year  period.  Many  of  the  rest  were  never  plowed, 
while  some  were  plowed  in  part  of  the  years  only. 

There  was  no  difference  in  average  yield  between  the  plowed  and 
the  unplowed  groves.  The  occasionally  plowed  groves  had  an  average 
yield  15  per  cent  less  than  the  other  groups.  It  would  be  difficult 
to  say  whether  the  irregularity  of  plowing  actually  caused  the  de- 
crease in  yields  or  whether  the  growers  were  attempting  to  correct 
some  other  unfavorable  condition  by  modifying  their  orchard  pro- 
gram. The  evidence  seems  to  point  to  the  former — that  is,  it  appears 
that  irregular  cuttings  of  roots  by  occasional  plowing  may  be  ex- 
pected to  temporarily  reduce  yields,  while  regular,  annual  plowing 
does  not  have  such  an  effect. 

It  is  of  interest  that  in  certain  communities  no  one  plows  because 
of  an  opinion  that  to  do  so  reduces  yields,  while  in  adjoining  com- 
munities such  a  prejudice  is  not  held  and  everyone  plows. 

Neither  policy  has  any  immediate  effect  on  average  yields  of  a 
large  group  of  groves,  although  there  are  apparently  a  few  authentic 
cases  in  which  plowing  at  the  wrong  time  in  relation  to  fruit  setting 
or  to  soil  moisture  has  resulted  in  a  heavy  drop  of  fruit.  It  seems, 
therefore,  that  in  general  good  farming  practice  should  dictate  the 
policy  to  be  followed.     All  the  successful  schemes  of  crop  rotation 


24  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

include  a  deeply  tilled  crop  and  the  use  of  a  legume;  plowing  and 
green-manuring  are  fundamental  features  of  world  agriculture.  They 
can  both  be  used — and  combined — in  citrus  culture.  The  evidence  of 
this  survey  of  growers'  experience  is  that  plowing  down  a  winter 
green-manure  crop  each  spring  certainly  does  no  harm.  Agricultural 
history  teaches  that  such  a  practice  is  one  approach  to  permanent 
agriculture.  Therefore,  wherever  possible,  winter  legumes  should  be 
grown  and  plowed  down  to  a  greater  depth  than  the  subsequent 
summer  cultivation.  It  should  be  remembered,  however,  that  the 
cover  crop  may  require  some  extra  irrigation  water,  especially  in 
years  of  light  rainfall. 

THE  "LIMIT  OF  PEOFITABLE  CULTIVATION" 

The  produce  of  a  given  piece  of  land  cannot  be  doubled,  trebled, 
quadrupled,  and  so  on  indefinitely,  by  merely  doubling,  trebling,  and 
quadrupling  the  amount  of  labor  and  capital  expended  in  its  culti- 
vation. In  the  data  of  this  survey,  the  increases  in  yields  following 
additional  nitrogen  applications  grow  constantly  less  until  a  point 
is  reached  where  no  increase  in  crops  is  experienced.  Diminishing 
physical  returns  from  each  additional  application  of  nitrogen  are 
found  beginning  with  the  first  class  into  which  the  data  are  divided. 

The  ' '  limit  of  profitable  cultivation  "  is  in  general  a  different  thing 
from  either  the  point  of  diminishing  physical  returns  or  that  of  most 
productive  use.  It  represents  the  most  profitable  use  of  the  combined 
factors  of  production.  It  is  usually  reached  after  diminishing  returns 
set  in  and  before  the  most  productive  use  is  realized.  Occasionally 
it  goes  outside  of  these  limits. 

Profits,  as  ordinarily  conceived  and  as  used  in  this  discussion,  may 
be  divided  into  two  separate  concepts: 

(1)  Returns  on  capital  invested; 

(2)  Payment  for  assuming  the  risk  of  doing  business. 

No  attempt  will  be  made  in  this  discussion  to  separate  these  two 
factors  but  both  will  be  included  in  the  term  "profit,"  and  will  be 
referred  to  in  terms  of  an  interest  rate  on  the  investment. 

Agricultural  lands  are  capitalized  in  proportion  to  their  earning 
power.  The  flexibility  of  this  capitalization,  with  changes  in  earning 
power,  was  well  illustrated  by  the  rapid  rise  in  the  selling  prices  of 
corn-belt  farms  during  the  war  period  of  high  prices  for  agricultural 
products,  followed  by  the  rapid  slump  in  land  values  when  the  prices 
paid  for  agricultural  products  dropped  more  rapidly  than  the  cost  of 
their  production.     Ordinarily  the  demand  for  agricultural  lands  is 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  25 

strong  enough  to  keep  their  capitalization  high  in  proportion  to  their 
earning  power.  Farm  management  surveys  made  in  various  states 
indicate  that  on  the  average,  farms  are  earning  only  about  3  per  cent 
on  the  farmers'  estimate  of  their  sales  value. 

Among  other  data  secured  from  growers  in  the  present  survey 
were  their  estimates  of  the  present  sales  value  of  the  groves,  as  well 
as  their  estimate  of  the  actual  cost  of  production  of  fruit  during  the 
past  five  years.  These  figures  have  been  compared  with  the  average 
production  in  pounds  of  fruit  for  the  different  districts  and  with  the 
average  selling  prices  of  citrus  fruits  on  the  trees  for  the  past  ten 
years.  The  resulting  figure  shows  an  average  profit  of  between  3% 
and  41/2  per  cent  for  the  groves  included  in  the  survey. 

In  any  consideration  of  the  relative  profitableness,  either  of  indi- 
vidual orchards  or  of  varying  cultural  practices,  at  least  four  factors 
must  be  taken  into  consideration,  namely : 

(1)  Cost  of  production  per  pound  of  fruit; 

(2)  Sales  price  for  the  fruit; 

(3)  The  amount  of  fruit; 

(4)  The  value  of  the  land  upon  which  returns  must  be  paid. 
The  margin  between  the  cost  of  production  and  the  selling  price  per 
pound,  multiplied  by  the  yield  gives  the  total  net  return.  This  figure 
divided  by  the  valuation  of  the  orchard  gives  the  per  cent  of  return 
on  this  valuation.  Each  of  these  factors  plays  so  important  a  part 
in  the  answer  to  the  final  question  of  net  profit  that  it  is  impossible  to 
consider  any  one  of  them  without  assuming  a  fixed  point  or  a  regular 
schedule  for  each  of  the  others. 

Cost  of  Fruit  per  Pound 

Probably  the  cost  of  producing  a  pound  of  fruit  has  most  fre- 
quently been  used  as  a.  measure  of  the  comparative  merits  of  two 
orchard  enterprises.  This,  however,  is  by  no  means  a  sure  guide,  as 
is  illustrated  by  the  figures  in  table  16,  comparing  data  for  oranges 
from  two  southern  California  districts. 

If  an  interest  charge  on  an  estimated  valuation  of  groves  in  each 
of  the  above  districts  has  been  included  in  the  cost  of  production,  the 
costs  per  box  would  measure  the  relative  accuracy  of  such  estimated 
valuations.  If  the  costs  per  box,  including  interest,  were  equal,  the 
valuations  used  would  be  relatively  equal.  If  one  group  showed  a 
higher  cost,  including  interest,  than  another,  its  estimated  value  would 
be  relatively  too  high.  Except  when  used  in  this  manner,  the  cost 
of  production  per  box  is  important  as  only  one  factor  in  an  equation 
and  is  not  of  itself  a  measure  of  the  merits  of  the  enterprise. 


26 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  16 

Illustration  of  the  Difference  w  Valuation  per  Acre  of  Orange  Orchards 

that  May  Exist  Even  when  Cost  of  Production 

per  Pound  of  Fruit  is  the  Same 

(One  acre) 


Items 

District  No.  1 

District  No.  2 

Total  cost  of  production  to  time  of  harvesting 

Yield  in  packed  boxes 

$155.00 
143 

1.08 

185.00 

30.00 

lOOO.OO 

$230.00 

225 

Cost  per  box  for  fruit  on  trees 

1.07 

Returns  for  fruit  on  trees 

295.00 

Farmer's  return  on  capital 

65.00 

Capitalization  that  will  allow  3  per  cent  return 

2165.00 

Net  Returns  per  Acre 

Table  17  reports  the  data  collected  in  the  survey  that  are  neces- 
sary to  figure  the  net  returns  per  acre  for  orange  and  lemon  groves 
separately  in  each  of  the  climatic  zones.  The  per  acre  yields  and 
operating  costs  are  the  averages  from  the  survey  reports  from  each 
zone.  The  prices  used  are  the  approximate  ten-year  average  prices 
(1912-1922)  paid  to  the  growers  for  fruit  on  the  trees  by  the  asso- 
ciations affiliated  with  the  California  Fruit  Growers'  Exchange. 


TABLE  17 

Net  Eeturns  per  Acre  for  Oranges  and  Lemons  Separately  in  Each  of  the 
Climatic  Zones,  with  Present  Cultural  Practices 


Yield 

Price1 

Gross 
returns 

Costs2 

Zone 

Packed 
boxes 

Ten-year 

average  price 

per  packed 

box 

Total 

operating  costs. 

No  interest 

included 

Net 
returns 

Oranges : 

Coastal 

244 
230 
192 

$1.50 
1.40 
1.30 

$366.00 
322.00 
250.00 

$230.00 
229.00 
188.00 

$146.00 

Intermediate 

93.00 

Interior 

62.00 

Lemons: 

Coastal 

310 
300 
236 

$1.30 
1.30 

1.30 

$404.00 
390.00 
307.00 

$265.00 
274.00 
230.00 

$139.00 

Intermediate  

116.00 

Interior 

77.00 

1  Valencias  have  sold  for  about  45  cents  per  packed  box  more  than  navels  on  a  ten-year  average. 
On  a  five-year  average  this  difference  has  been  less  than  half  as  much.  The  different  prices  assigned 
to  the  several  zones  make  allowance  for  the  different  proportion  of  valencias,  on  the  basis  of  the  ten 
year  difference  in  price. 

2  Differences  in  costs  between  the  zones  are  due  in  the  main  to  different  fertilizer  practices,  different 
pest  control  needs,  and  different  tax  assessments. 


Bulletin    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


27 


Net  returns  per  acre  are  indicative  of  relative  valuations  of 
orchards  on  an  acreage  basis.  This  criterion  of  value  must  not  be 
confused,  however,  with  the  factor  of  net  returns  on  the  investment. 
Financially  the  citrus  grower  is  interested  in  the  relation  which  exists 
between  net  returns  per  acre  and  the  present  exchange  value  of  that 
acre. 

Net  Returns  on  the  Investment 

The  averages  of  growers'  own  estimates  of  the  present  exchange 
value  of  their  groves  are  widely  different  for  the  three  climatic  zones. 
In  table  18  the  returns  per  acre  are  converted  into  terms  of  returns 
on  a  given  financial  investment.  The  growers'  estimate  of  valuation 
is  the  datum  from  which  the  calculations  commence.  The  illustration 
is  based  on  an  investment  of  $20,000  because  that  is  the  approximate 
average  (modal)  value  of  California  citrus  orchards. 


TABLE  18 

Average  Net  Eeturns  on  the  Investment  for  Oranges  and  Lemons  Separately 

in  Each  of  the  Climatic  Zones,  with  Present  Cultural  Practices 


Growers' 
estimated 
valuation 

Net 
returns 
obtained 

on 

estimated 

value 

What  to  expect  from  a  $20,000  investment  at 
growers'  estimated  value 

Zone 

Acres 
pur- 
chased 

Net 
returns 

to 
capital 

xHours 
of  labor 
that 
owner 
might 
perform 

iValue 

of  owners 

labor 

at 

30  cents 

Total 
possible 
income 
to  labor 

and 
capital 

Oranges : 

Coastal 

$4100 
2680 
1540 

per  cent 
3.56 
3.48 
4.02 

4.9 

7.5 

13.0 

$711 
696 
804 

640 

975 

1690 

$194 
292 
506 

$905 

Intermediate 
Interior 

.    988 
1310 

Lemons : 

Coastal 

$3500 
2600 
1800 

4.00 
4.45 

4.28 

5.7 

7.7 

11.1 

$800 
890 
835 

865 
1230 
1780 

$260 
370 
535 

$1060 

Intermediate 
Interior..'. 

1260 
1370 

1  These  columns  refer  only  to  man  labor  and  are  based  on  the  average  experience  of  75  growers  who 
furnished  diary  reports  to  the  writer  from  1915  to  1918.  The  average  was  130  hours  manual  labor  per 
acre  annually  on  oranges  and  160  hours  on  lemons.  This  does  not  include  picking  or  hauling  of  fruit, 
or  fumigating  labor.  The  difference  between  oranges  and  lemons  is  due  mainly  to  different  pruning 
requirements. 

The  Most  Profitable  Use  of  Nitrogen 

Changes  in  orchard  practices  will  ordinarily  modify  both  the 
operating  costs  per  acre  and  the  yields.  The  complex  reaction  of 
such  changes  on  the  net  returns  on  the  investment  can  best  be  illus- 
trated by  considering  the  most  profitable  use  of  nitrogen  in  fertilizing 
orange  groves. 


28 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  per  acre  yields  of  oranges  with  various  applications  of  nitro- 
gen are  recorded  in  table  10.  The  operating  costs  for  each  climatic 
zone  when  the  average  amount  of  nitrogen  is  used  are  given  in  table 
17.  These  costs  may  be  varied  for  different  nitrogen  applications 
on  the  assumption  that  all  other  operations  remain  constant. 

The  most  profitable  use  of  fertilizer  will  also  depend  in  part 
on  the  price  at  which  the  fruit  may  be  sold.  In  order  to  make  the 
example  complete,  the  effect  of  two  different  selling  prices  may  be 
considered — one  a  little  below  the  ten-year  average  and  the  other 
somewhat  above  it. 

Table  19  presents  a  summary  of  the  data  necessary  to  determine 
the  most  profitable  use  of  nitrogen  under  the  conditions  outlined 
above.     Figure  4  presents  the  same  data  graphically. 


TABLE  19 

The  Most  Profitable  Fertilizer  Practice  with  Two  Different 

Prices  of  Fruit 

Oranges 


Nitrogen 

in 
pounds 
per  acre 

Yield  in 
thousands 
of  pounds 

per  acre 

Total  cost 
per  acre 

varied  by 

nitrogen 

used 

Net  returns 
per  acre 

Net  returns  on 
$20,000  investment 

Zone 

With 
price  of 
l^c  for 
fruit  on 

trees 

With 
price  of 
2Mc  for 
fruit  on 

trees 

With 
price  of 
l^c  for 
fruit  on 

trees 

With 
price  of 
2H.c  for 
fruit  on 

trees 

Coastal 

350 

27.9 

$360 

$59 

$336 

$290 

$1645 

300 

26.3 

330 

65 

327 

319 

1605 

250 

25.0 

300 

75 

325 

368 

1590 

200 

22.2 

270 

72 

285 

353 

1395 

150 

20.7 

240 

71 

277 

348 

1355 

100 

18.3 

210 

65 

247 

319 

1210 

50 

14.8 

180 

42 

190 

206 

930 

Intermediate 

350 

25.0 

$350 

$25 

$275 

$188 

$2060 

300 

23.6 

320 

34 

270 

248 

2010 

250 

22.4 

290 

40 

270 

300 

2010 

200 

20.0 

260 

46 

240 

345 

1800 

150 

18.5 

230 

48 

232 

360 

1740 

100 

16.5 

200 

48 

212 

360 

1580 

50 

13.6 

170 

14 

170 

105 

1275 

Interior 

350 

22.0 

$340 

$-10 

$205 

$-130 

$2660 

300 

20.4 

310 

-  4 

210 

-  52 

2730 

250 

19.7 

280 

14 

212 

182 

2760 

200 

17.6 

250 

16 

190 

208 

2470 

150 

16.3 

220 

25 

187 

325 

2430 

100 

14.4 

190 

26 

170 

338 

2210 

50 

11.7 

160 

16 

132 

208 

1730 

Bulletin    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY 


29 


A  cost  of  $30  for  50  pounds  of  nitrogen  may  seem  excessive.  It 
is  based  on  the  high,  prices  paid  for  fertilizers  during  the  five  years 
included  in  this  study.  The  assumption  is  also  made  that  at  least 
one-half  of  the  nitrogen  comes  from  bulky  organic  manures — gen- 
erally an  expensive  source  when  nitrogen  alone  is  considered.  Of 
course  the  figure  used  includes  all  costs  of  application.  When  nitro- 
gen can  be  purchased  for  less  money  the  details  of  the  example  will 
be  changed. 


Met    Return 
On    a     ^20,000 
Investment 


The     Host   Profitable    ferfi/izer    'Practice 
With    Two    Different     Fruit     Prices 


Interior  Zone 


ZSOO 


*/,soa 


When  the   Price 

of  fruit  is 

22  *  Per  Pound 


A  OOO. 


*soo. 


When  the   Price 
of  Trail    is 
H  "  Per  Pound 


/ 

/ 

/ 
/ 
/ 

.^-  — 

—  • 

_/ 

^' 

Intermediate    Zone 

Coastal   Zone 

s 

y 

X"* 

^ 

■ — Coasro/    Zone . 

i 

i 

i 

~  ~  ~~  ~  -*  x     Inter medicrTe~~Z6ne~ 
1      ""->       1                        1 

SO 


too 


150  ZOO 

A/itrooen     ~-  Pounds 


/SO 
T>er  Acre. 


'*»*££*> 


Fig.  4.     Illustrating  tables  19  and  20. 

Two  deductions,  at  least,  may  be  drawn  from  the  financial  sum- 
maries submitted.  In  the  first  place,  heavier  fertilization  is  more 
profitable  when  prices  are  high  than  when  they  are  low.  If  there 
were  assurance  of  high  prices  growers  generally  would  be  justified 
in  fertilizing  almost  to  the  limit  of  maximum  physical  production. 
Certainly  present  applications  then  could  be  profitably  doubled. 

In  the  second  place,  when  prices  are  low,  the  high  yielding  zones 
(coastal  and  intermediate)  have  a  higher  potential  earning  power  on 
the  capital  invested  than  the  interior  zone.  When  prices  are  high, 
on  the  other  hand,  the  advantage  is  definitely  in  favor  of  the  interior 


30  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

district.     The  summary,  in  table  20,  of  the  net  returns  for  the  most 
profitable  fertilizer  group  brings  out  this  point, 

TABLE  20 
Average  Net  Eeturns  on  $20,000  Investment,  for  the  Most  Profitable 

Fertilizer  Group 

Fruit  l^c  Fruit  2^c 

Coastal $368.00  $1645.00 

Intermediate 360.00  2060.00 

Interior 338 .00  2760 .00 

The  figures  presented  above  may  have  little  permanent  value, 
except  as  illustrations  of  a  method  of  analyzing  the  citrus  business. 
They  are  based  entirely  on  present  land  valuations,  present  operating 
costs,  and  past  selling  prices  for  fruit.  As  any  of  these  factors 
change,  the  final  result  in  net  income  will  also  change.  The  financially 
successful  citrus  grower  of  the  future  will  be  the  one  who  most  accu- 
rately anticipates  changes  in  any  of  the  factors  and  acts  upon  such 
anticipation.  Obviously,  recommendations  as  to  the  "best"  fertilizer 
practice  to  follow  can  be  made  only  when  all  the  factors -considered 
above  are  known. 

COST  OF  DEVELOPING  CITRUS  ORCHARDS 

It  has  been  stated  above  that  agricultural  land  is  capitalized  on 
its  earning  power,  and  figures  have  been  submitted  which  indicate 
that  average  orange  groves  are  paying  net  returns  of  4  per  cent  on 
$3650  per  acre  near  the  coast  and  on  $1540  in  the  interior. 

It  is  equally  true  that  when  sufficient  time  is  allowed  for  the 
thorough  establishment  of  an  industry  the  value  of  a  single  unit, 
such  as  an  orchard,  will  closely  approximate  the  cost  of  establishing 
such  a  unit.  This,  of  course,  does  not  apply  where  partial  or  com- 
plete monopoly  exists.  It  is  clear,  however,  that  the  orange  industry 
of  California  cannot  be  considered  a  monopolized  industry  at  present, 
for  there  is  much  more  good  land  in  sight  than  there  is  good  market. 
The  past  ten  years  have  seen  extensive  new  plantings  of  oranges  along 
the  Gulf  coast  and  in  Florida,  while  California  acreage  has  increased 
relatively  slowly.  The  failure  of  orange  prices  to  rise  during  the 
war  period  in  keeping  with  other  prices  is  another  indication  of  the 
lack  of  monopolistic  character  in  the  industry. 

In  consequence  of  this  general  situation,  the  value  of  orange 
groves  should  closely  approximate  the  value  of  land  for  other  farm 
crops  plus  the  cost  of  bringing  an  orchard  into  bearing. 

In  the  coastal  districts  prospective  citrus  plantings  must  compete 
with  bean  lands,  which  are  often  priced  at  $800  to  $1200  per  acre 


BULLETIN    374]  ORCHARD   PRACTICES   IN    THE    CITRUS    INDUSTRY  31 

for  Lima  bean  production..  In  the  interior  the  competition  is  with 
deciduous  fruits,  alfalfa  or  grain  hay  and  a  few  other  crops,  and 
here  land  with  water  is  priced  at  about  $400  per  acre. 

During  the  five  years  1914-1919  the  writer  summarized  the  cost 
of  developing  orange  groves  on  nearly  one  hundred  different  projects 
in  the  interior  districts  varying  in  size  from  five  acres  to  100  acres. 
These  figures,  together  with  the  original  land  values  just  cited,  have 
been  used  to  construct  the  following  table  (table  21)  on  cost  of  estab- 
lishing a  young  grove.  In  table  22  the  detail  figures  from  table  21 
are  summarized,  together  with  credits  to  be  expected  from  sales  of 
fruit,  and  interest  on  the  total  money  in  the  capital  account. 

TABLE  21 

Cost  per  Acre  of  Developing  Young  Orange  Orchards  in  Riverside  and 
San  Bernardino  Counties,  1914-1919 

Land  with  water,  ready  to  plant $400.00 

Trees  planted 100.00 

Pipe  lines  (allowing  one  line  across  a  square  ten  acres) 25 .  00 

$525.00 

Cost  per  year  for  first  four  years: 

Labor  and  team  work $  20.00 

Fertilizer  and  cover  crop 6.00 

Taxes 5.00 

Water  charges 14.00 

Miscellaneous 7.00 

Administration,  superintendent,  etc 8.00 

$60.00  $240.00 

Cost  per  year,  5th-6th  years,  inclusive : 

Labor  and  team  work $25  .00 

Fertilizer  and  cover  crop 20.00 

Taxes 7.00 

Water 20.00 

Pest  control 6.00 

Miscellaneous 8.00 

Administration *. 9.00 

$95.00  $190.00 

Cost  7th  to  10th  years,  inclusive : 

Labor  and  team  work $35  .00 

Fertilizer 40.00* 

Taxes 11.00 

Water 20.00 

Pest  Control 10.00 

Miscellaneous 12.00 

Administration 12.00 

$140.00  $560.00 

Total  operating  costs  for  first  ten  years $1515  .00 

*  10th  year,  $20.00  additional  fertilizer. 


32 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  22 

Developing  Orange  Orchards — Interior  Zone.     Accumulated  Cost  with 
Interest  Charged  and  Sales  Credited 


Operating 
cost 


Credit 
for  sales 


Net 
cost 


Net 
credit 


Interest 

at  4 
per  cent 


Total 
accumu- 
lated 
cost 


Original  cost.. 

First  year 

Second  year.. 

Third  year 

Fourth  year.. 

Fifth  year 

Sixth  year 

Seventh  year 
Eighth  year... 
Ninth  year.... 
Tenth  year... 


$525 

60 

60 

60 

60 

95 

95 

140 

140 

140 

160 


$39 
78 
110 
156 
195 


$525 
60 
60 
60 
60 
95 
56 
62 
30 


$16 
35 


$21.00 
24.24 
27.61 
31.11 
34.76 
39.94 
43.68 
48.00 
51.13 
52  53 


$525.00 

606. Q0 

690.24 

777.85 

868.96 

998.72 

1094.66 

1200.34 

1278.34 

1313.47 

1331  00 


These  figures  on  the  cost  of  developing  orange  orchards  approach 
closely  to  the  grower's  average  estimate  of  values  of  mature  groves 
in  the  interior  districts.  Presumably  the  higher  price  of  land  and 
consequent  higher  interest  charges  will  produce  a  similar  agreement 
with  the  orchard  values  in  the  other  districts.  The  orange  industry 
in  California  has  apparently  reached  a  point  where  rapid  increase  in 
plantings  is  not  to  be  expected  because  the  land  is  just  as  valuable 
for  other — and  possibly  less  hazardous — purposes. 


CONCLUSIONS 

Citrus  groves  produce  more  fruit  per  acre  near  the  coast  than  in 
the  interior.  They  do  not,  on  the  other  hand,  return  higher  net  profits 
on  present  valuations. 

The  middle  range  of  soils  (by  texture),  which  are  mainly  used 
for  citrus  planting  in  southern  California,  are  about  equally  well 
adapted  to  this  purpose.  Very  sandy  soils  and  heavy  clay  adobes  are, 
in  general,  less  productive  of  citrus  fruit. 

Citrus  trees  may  be  expected  to  increase  in  average  yield  until  at 
least  35  years  of  age.  There  is  no  evidence  that  40-year-old  trees 
produce  less  fruit  because  of  their  age. 

Nitrogen  and  bulky  organic  manures  appear  to  be  the  only  fer- 
tilizing constituents  that  need  to  be  supplied  for  successful  citrus 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  33 

production  in  southern  California.  About  half  the  nitrogen  now 
applied  to  citrus  groves  is  carried  in  bulky  organic  manures. 

Average  yields  increase  with  increased  applications  of  nitrogen 
uutil  about  350  pounds  per  acre  are  applied.  The  point  of  most 
profitable  use  depends  on  several  variable  factors.  Examples  are 
presented  illustrating  the  interplay  of  these  factors.  Mottling  appar- 
ently results  from  excessively  heavy  applications  of  nitrogen. 

Orchards  winter-cover-cropped  gave  higher  yields  than  clean- 
cultivated  orchards  which  received  the  same  quantities  of  bulky 
organic  manures.  When  grown  for  several  years,  summer-cover 
crops  seem  to  result  in  poor  tree  condition.  The  use  of  cover  crops 
does  not  seem  to  lessen  the  need  of  nitrogen.  Rotation  of  leguminous 
cover  crops  is  desirable. 

Irrigation  has  an  important  effect  on  yields.  In  general,  less 
water  is  and  should  be  used  near  the  coast  than  in  the  interior.  The 
interval  between  irrigations  should  be  longer  in  the  cooler  coastal 
districts. 

The  citrus  industry  seems  to  have  reached  a  stage  of  stability,  for 
there  is  a  close  agreement  between  (1)  the  grower's  estimate  of  the 
exchange  value  of  groves,  (2)  the  capitalization  of  the  present  earn- 
ing power  of  groves,  and  (3)  the  cost  of  developing  new  groves  from 
land  purchased  at  its  present  valuation  for  other  crops. 


APPENDIX 

HOMOGENEITY  AND  ACCURACY  OF  THE  DATA 

In  order  to  make  the  individual  grove  records  as  nearly  comparable  as  pos- 
sible, the  orchard  practices  of  five  specific  years  (1917-18  to  1921-22)  were  taken 
in  all  cases.  For  instance,  the  fertilizer  usage  for  these  five  years  was  listed,  and 
the  average  amount  applied  each  season  was  found  for  each  individual  grove.  This 
average  should  represent  normal  practice  rather  closely. 

In  the  case  of  yields  only  the  three  middle  years  of  the  five  were  averaged. 
The  1917-18  crop  was  affected  so  seriously  by  heat  in  certain  sections  that  it  was 
discarded,  and  similarly,  the  1921-22  yields  were  omitted  because  of  the  varying 
degree  of  frost  injury. 

The  yield  figures  were  always  checked  with  the  packing-house  statements,  and 
the  irrigation  figures  with  the  irrigation  companies'  records.  Fertilizer  records 
could  not  be  so  easily  checked,  but  because  of  the  relative  importance  of  this  item 
a  written  memorandum  of  amounts  applied  had  usually  been  kept  by  the  grower. 

The  total  number  of  groves  actually  used  in  the  calculations  was  600.  The 
effect  of  the  number  of  groves  upon  the  class  averages  is  indicated  by  the  follow- 
ing tables  (23  and  24)  summarized  at  different  times  during  the  investigation. 


34 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


TABLE  23 

Effect  of  Nitrogen  on  Yields 
Relative  yields 


Pounds  N  per  acre 

1st  50  groves 

1st  300  groves 

Total  600  groves 

350 

100 

100 

100 

300 

102 

96 

94 

250 

95 

91 

89 

200 

86 

83 

78 

150 

82 

70 

73 

100 

74 

61 

66 

50 

53 

57 

57 

TABLE  24 

Effect  of  Age  of  Trees  on  Yields 

Relative  yields 


Age 

1st  50  groves 

1st  300  groves 

Total  600  groves 

35 

100 

100 

100 

30 

99 

98 

97 

25 

97 

95 

94 

20 

93 

92 

92 

15 

83 

85 

89 

10 

46 

57 

60 

METHODS  USED  IN  ANALYSIS 

In  the  handling  of  survey  data  the  method  usually  followed  has  been  to  sort 
the  records  into  classes  according  to  some  one  variable  factor  and  to  deterimne 
the  average  yields  or  profits  for  each  class.  The  differences  between  these  aver- 
ages have  been  taken  as  measures  of  the  effect  of  changes  in  the  variable  factor, 
on  the  assumption  that  the  law  of  large  numbers  has  equalized  any  effects  of  other 
factors.  Sometimes,  when  it  was  obvious  that  secondary  factors  were  affecting 
the  average  of  one  class  to  a  greater  degree  than  that  of  another  class,  the  records 
have  been  further  divided  into  sub-classes.  Frequently,  however,  this  method  has 
reduced  the  number  of  records  in  the  sub-classes  to  a  point  where  the  probable 
error  of  the  average  made  the  differences  between  class  averages  of  no  significance. 

Confidence  in  the  class  averages  as  ordinarily  obtained  may  be  greatly  strength- 
ened if  correlation  coefficients  are  determined  between  the  variables  that  are  being 
compared.  When  inter-relations  exist  between  several  variables,  the  net  agree- 
ment between  any  two  of  them  may  be  determined  by  the  use  of  the  partial  cor- 
relation method.  This  technique  has,  been  successfully  applied  to  an  analysis 
of  survey  data  on  the  fattening  of  baby  beef.7 

Correlation  coefficients  do  not  take  the  place  of  group  averages  for  they  do 
not  measure  the  value  of  one  variable  which  corresponds  to  a  particular  value  of 


7  The  theory  of  correlation  as  applied  to  farm-survey  data  on  fattening  baby 
beef,  by  R.  H.  Tolley.    U.  S.  D.  A.,  Bull.  504.  pp.  1-14,  1917. 


Bulletin  374]  orchard  practices  in  the  citrus  industry 


35 


another  variable.  What  they  do  measure  is  the  degree  to  which  movements  of  one 
variable  are  accompanied  by  movements  in  another  variable,  and  also,  whether  the 
movements  are  in  the  same  or  in  opposite  directions.  When  correlation  coefficients 
are  high,  differences  in  class  averages  are  of  significance ;  when  the  coefficients  are 
low,  differences  in  averages  are  probably  due  to  imperfect  sampling.  In  the 
present   study  the  correlation  technique  has  been  utilized  extensively. 

Classification  of  the  Variables 

The  factors  upon  which  information  was  gathered  fall  into  two  main  classes. 
Certain  of  the  variables,  such  as  amount  of  fruit  produced,  fertilizer  used,  irri- 
gation water  applied,  and  so  on,  can  be  measured  quantitatively.  The  agreement 
between  changes  in  any  of  these  things  can  be  studied  statistically  by  recognized 
methods  for  quantitative  variables.  Other  items,  such  as  climatic  environment, 
and  soil  type,  cannot  be  classified  quantitatively  and  are  therefore  subject  only 
to  those  types  of  analyses  that  apply  to  qualitative  variables. 

The  list  of  variables  upon  which  data  were  obtained  and  their  division  into 
statistical  classes  is  given  in  table  25. 


TABLE  25 
List  of  Variables  upon  which  Data  were  Obtained 


Quantitative  variables. 


Classes 

Variable 

No. 

Centering 
at 

Centers  of 
extreme  classes 

1.    Age  of  trees 

7 
8 

5 

3 

7 
10 

multiples  of  5  years 
multiples   of   50 

pounds  per  acre 
multiples  of  5  tons 

per  acre 
limits   dependent 
upon  climatic  dis- 
trict, 
weekly  intervals 
multiples  of  20  per 
cent  of  the  aver- 
age yield 

10  and  40  years 

2.    Average  annual  application  of 
nitrogen 

50  and  400  pounds 

3.    Average  annual  application  of 
manure 

5  and  25  tons 

4.    Average  annual  application  of 
irrigation  water 

light  and  heavy  ir- 
rigation 

2  and  8  weeks 

5.    Interval  between  irrigations  dur- 
ing summer 

6.    Yields 

40  per  cent  and  200 

7.    Costs 

per  cent 

Qualitative  variables: 


Classes 

No. 

Designations 

1.  Climatic  environment 

2.  Soil  types 

3 
3 
6 
6 

coastal,  intermediate,  interior 
light,  medium,  heavy 

3.  Tillage 

number  of  years  plowed  during  past  five 

4.  Cover  crops 

number  of  years  grown  during  past  five 

36  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Because  of  the  separation  of  variables  into  qualitative  and  quantitative  groups 
it  became  desirable  to  use  the  statistical  methods  both  of  association  and  cor- 
relation in  the  study  of  the  data.  Fortunately  for  the  subsequent  analysis  a  high 
degree  of  independence  was  found  to  exist  among  the  several  variables  in  each 
of  the  groups  of  the  qualitative  classifications.  That  is,  for  instance,  the  same 
proportions  of  heavily,  moderately,  and  lightly  fertilized  groves  were  found  in 
each  of  the  three  geographic  districts — and  similarly  on  each  of  the  three  soil 
types. 

Association  of  Variables 

Coefficients  of  association  were  determined  between  each  of  the  several  quali- 
tative classifications  of  climate,  soil,  tillage  and  cover  crop  and  the  quantitative 
variables  of  fertilizer  and  irrigation  practices  as  well.  The  standard  formula 
for  this  purpose  was  used: 

_AB>ab—Ab.aB 
Q^~  AB.ab  +  Ab.aB 
in  which  the  difference  between  the  cross  products  is  divided  by  the  sum  of  the 
cross  products.  In  the  formula  the  A 's  and  the  B  's  represent  the  possession  of  an 
attribute,  such  as  location  in  the  coastal  zone  (A)  and  on  heavy  soil  (B),  while 
the  a's  and  b's  stand  for  the  lack  of  that  particular  attribute.  In  complete  asso- 
ciation Q  =■  1  because  the  second  term  in  both  numerator  and  denominator  is  0. 
In  complete  disassociation  Q  —  —  1  because  the  first  term  in  both  numerator  and 
denominator  is  0.     In  complete  independence  Q  =  0  because  the  numerator  is  0. 

Nearly  all  of  the  coefficients  thus  determined  lay  between  — .15  ±  .06  and 
-f-  .15  ±  .06,  showing  a  high  degree  of  independence.  The  following  exceptions 
may  be  noted: 

1.  There  is  a  higher  proportion  of  young  groves  (ten-year  class)  in  the  coastal 

zone  than  in  the  rest  of  the  population,  for  when 

A  =  all  groves  in  the  coastal  zone,  and 

B  =  all  groves  in  the  ten-year  class, 

a  =  all  groves  not  in  the  coastal  zone,  and 

b  =  all  groves  not  in  the  ten-year  class, 
then    Q  =  .5  ±  .04,    indicating    a    greater    association    between    the    B  's 
and  A  's  than  between  the  B  's  and  a 's. 

This  is  of  no  significance  to  the  subsequent  analysis,  because  all  the 
groves  in  the  ten-year  class  were  eliminated. 

2.  The  amount  of  water  used  in  the  coastal  zone  is  more  nearly  uniform  than 

in  the  other  zones,  for  when 

A  =  all  groves  in  the  coastal  zone,  and 

B  ==  all  groves  using  the  ' '  usual '  's  amount  of  water 

a  =  all  groves  not  in  the  coastal  zone,  and 

b  =  all  groves  using  other  than  the  ' '  usual ' '  amount  of  water, 
then  Q  =  .6  ±  .03,  indicating  that  a  higher  proportion  of  groves  in  the 
coastal  zone  (A)  used  the  usual  amount  of  water  (B)  than  was  the  case 
in  the  other  zones  (a).  This  is  equivalent  to  saying  that  there  is  a  more 
pronounced  mode  of  irrigation  usage  in  the  coastal  district  than  else- 
where. The  range  of  usage  is  almost  as  great  in  the  coastal  zone  but 
the  non-modal  classes  are  not  so  well  represented. 


s  For  explanation  of  this  term  see  page  22. 


BULLETIN    374]  ORCHARD   PRACTICES    IN    THE    CITRUS    INDUSTRY  37 

3.  There  is  a  higher  proportion  of  heavily  fertilized  groves  in  the  intermediate 
zone  than  in  the  other  zones,  for  when 

A  =  all  groves  in  the  intermediate  zone, 

B  =  all  heavily  fertilized  groves, 

a  c=  all  groves  not  in  the  intermediate  zone, 

b  —  all  groves  not  heavily  fertilized, 
then  Q  =  .4  ±  .04,  showing  that  relatively  more  of  the  heavily  fertilized 
groves   (B)  were  in  the  intermediate  zone   (A)  than  elsewhere   (a). 

Allowance  was  partially  made  for  this  fact  in  the  figures  given  in 
table  10  by  using  the  different  observed  mean  nitrogen  applications  for 
the  three  zones  as  the  datum  from  which  the  relative  yields  were  converted 
into  pounds. 

Averages  Based  on  Law  of  Large  Numbers 

Inasmuch  as  the  independence  between  the  several  qualitative  variables  is  so 
high,  the  law  of  large  numbers  should  act  so  that  the  mean  figures  for  the  different 
classifications  are  significant  of  the  effect  of  the  qualitative  variables.  For  in- 
stance, mean  yields  for  the  three  climatic  zones  should  be  an  approximate  measure 
of  the  effect  of  climate  on  production,  and  mean  yields  for  the  three  soil  types 
should  be  a  fair  measure  of  the  adaptability  of  soils  to  citrus  production.  The 
other  variables,  being  independent  in  their  movements,  will  not  affect  such  means. 

After  ascertaining  the  essential  independence  of  the  variables,  the  mean  yield 
was  determined  for  each  class  in  the  several  quantitative  arrays  such  as  age  of 
trees,  amount  of  nitrogen,  manure,  and  water  used,  and  so  on.  These  figures 
represent  the  normal  effect  that  may  be  expected  to  accompany  changes  in  age  or 
in  orchard  practices. 

Partial  Correlation  Coefficients 

In  order  to  study  more  completely  the  complex  relations  between  the  quan- 
titative variables,  partial  correlation  coefficients  were  determined  on  six  variables, 
namely : 

(a)  Age  of  trees; 

(6)   Annual  application  of  nitrogen; 

(c)  Annual  application  of  manure; 

(d)  Annual  application  of  irrigation  water; 

(e)  Interval  between  irrigations; 
(/)   Yields. 

The  usual  formula  for  partial  correlation  was  employed,  in  which 

_  fab-cde  —  faf-cde  •  Hf-cdfi 
fab-cdef ' — ' 


V  (1  —  r^af.cde)  (1  — r^-cde) 

In  this  case  ra^.caef  represents  the  intensity  of  agreement  between  a  and  b  when 
their  separate  agreement  with  c,  d,  e,  and  /  is  taken  into  account. 

A  complete  list  of  the  net  correlation  coefficients  is  given  in  table  26.  The 
more  significant  net  and  partial  coefficients  are  commented  on  in  the  following 
paragraphs. 


38  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

The  correlation  coefficients  between  nitrogen  used,  and  yield  when  age  is  taken 
into  account,  are  a  measure  of  the  intensity  of  agreement  in  the  movements  of  the 
two  variables.  These  coefficients  are  all  high  in  relation  to  their  probable  errors. 
The  coefficients  are : 

Coastal  .612  ±  .03 

Intermediate  .624  ±  .03 

Interior  .690  ±  .03 

In  the  interior  and  intermediate  districts  there  is  a  tendency  to  fertilize  more 
heavily  as  the  groves  grow  older.  This  tendency  does  not  appear  in  the  coastal 
district.  The  correlation  coefficients  between  nitrogen  used  and  age  of  trees,  when 
the  items  of  manure  and  water  applied  and  intervals  between  irrigations  are  taken 
care  of,  are: 

Coastal  .112  ±  .06  """ 

Intermediate  .510  ±  .03 

Interior  .654  ±  .03 

The  net  coefficients  between  nitrogen  and  yields,  after  all  other  quantitative 
factors  are  taken  into  account,  are: 

Coastal  .372  ±  .04 

Intermediate  .626  ±  .03 

Interior  .543  ±  .04 

The  agreement  in  the  coastal  zone  is  slight,  in  part  at  least,  because  some  young 
groves  are  heavily  fertilized  while  the  soil  is  still  so  rich  that  it  will  not  respond. 
On  the  whole,  these  coefficients  are  high  enough  to  add  considerable  confidence  to 
the  figures  obtained  as  average  yields  following  different  nitrogen  applications. 

The  coefficients  between  amount  of  manure  used  and  amount  of  nitrogen  used 
indicate  that  increases  or  decreases  in  the  use  of  these  two  items  occur  together. 
This  is  to  be  expected  inasmuch  as  half  the  total  nitrogen  comes  from  manure  as 
an  average. 

It  appears  that  when  the  interval  between  irrigations  is  shortened,  the  total 
amount  of  water  used  in  the  season  is  increased.  This  has  resulted,  especially  in 
the  interior,  from  a  definite  attempt  on  the  part  of  growers  to  use  more  water  by 
furnishing  occasional  extra  applications. 

The  coefficients  between  yields  and  amount  of  water  used  are  low.  This  really 
bears  out  the  data  presented  in  table  15.  The  correlation  formula  used  is  that 
of  a  straight  line  fit,  while  the  effect  of  increased  applications  of  irrigation  water 
is  first  to  increase  and  later  to  decrease  yields.  Naturally  such  data  show  little 
agreement  with  a  straight  line  increase. 

Increased  applications  of  nitrogen  are  accompanied  by  increased  use  of  water 
in  the  interior  and  intermediate  zones.  Apparently  certain  farmers  are  working 
their  orchards  more  intensively  in  both  these  ways. 

On  the  whole,  the  correlation  coefficients  may  be  said  to  materially  increase 
the  confidence  in  the  significance  of  the  averages  presented  in  the  main  text  of 
the  bulletin. 


BULLETIN    374]  ORCHARD   PRACTICES   IN    THE    CITRUS    INDUSTRY 


39 


TABLE  26 

Net  or  Final  Correlation  Coefficients 
These  coefficients  measure  the  intensity  of  agreement  in  the  movements  of  the  two 
variables  concerned,  when  the  effect  of  the  four  other  factors  is  taken  into  account. 


Factors  compared 

Net  coefficients  for  each  zone 

separately 

Coastal 

Intermediate 

Interior 

Yield  and  frequency  of  irrigation 

Yield  and  amount  of  water  used 

Yield  and  amount  of  manure  used 

Yield  and  amount  of  nitrogen  used 

Yield  and  age  of  trees 

-.070±.05 

-.069±.05 

.234±.05 

.372  ±.04* 

.052±.05 

-.234±.05 
.019±06 
.082±.05 
.204±.05 

-.078±.05 
.285±04 
.026±.05 
.389  ±.04* 
.134±.05 

-.124±.04 

-.236±.04 
.204±.04 
.273±.04 
.626  ±.03* 
.150±04 

-.507±.03* 

-.330±.04 

.444  ±.03* 

-.108  ±.04 

-.283±.04 

.760  ±.02* 

.624±.03* 

.638  ±.03* 

.351±04 

.487  ±.03* 

-.020±.06 
.249±.05 
.054  ±.06 
.543  ±.04* 
.172±.05 

Frequency  of  irrigation  and  amount 
of  water  used 

-.688±.03* 

Frequency  of  irrigation  and  amount 
of  manure  used 

Frequency  of  irrigation  and  amount 
of  nitrogen  used 

.106±.05 
.937±.01* 

Frequency  of  irrigation  and  age  of 
trees 

Amount  of  water  used  and  amount 
of  manure  used 

-.673±.03* 
.436±.04 

Amount  of  water  used  and  amount 
of  nitrogen  used 

.665  ±.03* 

Amount  of  water  used  and  age  of 
trees 

.632  ±.03* 

Amount  of  manure  used  and  amount 
of  nitrogen  used 

.523  ±.04* 

Amount  of  manure  used  and  age  of 
trees.... 

Amount  of  nitrogen  used  and  age  of 
trees 

.316±.05 
.654  ±.03* 

♦Large  enough  in  proportion  to  probable  error  to  show  some  agreement  in  movement. 


Method  of  Combining  Groves  from  the  Three  Climatic  Zones 

The  effect  of  fertilizer  treatments  on  yields  appears  to  have  been  very  similar 
in  the  three  climatic  zones.  The  correlation  coefficients  between  yields  and  nitro- 
gen applied  when  age  of  trees  is  taken  into  account  are: 

Coastal  r=.612±.03 

Intermediate  r  =  .624  ±  .03 

Interior  -r=  .690  ±  .03 

These  coefficients  were  derived  from  the  formula 

fab  —  Tac  •  rbc 


fab-C — 


where 


V(l  —  r*ac)  (1  —  r%c) 


a  =  yield ; 

b  e=  amount  of  nitrogen  used ; 

c  =  age  of  trees. 


40 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 


Because  of  this  apparent  agreement  in  effect,  the  records  from  the  three  zones 
were  combined  to  give  the  law  of  large  numbers  a  better  chance  to  operate.  This 
was  accomplished  by  reducing  the  yield  of  each  grove  to  a  relative  of  the  average 
yield  in  its  zone.  The  yields  of  individual  orange  and  lemon  orchards  were  divided 
by  their  respective  average  yields.  Then  all  of  the  relatives  so  found  were  com- 
bined into  a  single  series  for  comparison  with  fertilizer  treatments.  For  this  pur- 
pose the  groves  were  grouped  according  to  the  total  amount  of  nitrogen  applied 
and  mean  yields  were  determined  for  each  group. 

In  figuring  the  amount  of  nitrogen  applied,  an  estimate  was  made  of  the 
amount  carried  in  any  bulky  organic  manures  used,  and  this  was  added  to  the 
amount  carried  in  commercial  fertilizer. 

Diminishing  Physical  Beturns  with  Increased  Use  of  Fertilizer 

There  is  usually  a  diminishing  physical  return  with  increasing  applications 
of  anything.  Many  series  of  fertilizing  and  feeding  data  form  the  terms  of 
decreasing  geometric  series,  in  response  to  the  law  of  diminishing  returns.9 
If  the  formula  for  the  sum  of  a  decreasing  geometric  series  is  applied  to  the  yield 
data  in  this  survey  and  the  several  points  corresponding  to  varying  nitrogen  appli- 
cations are  determined,  there  is  a  very  close  agreement  with  the  observed  facts. 
The  formula  referred  to  is 


S  = 


B 


(1  —  Rn) 


in  which  S  is  the  sum  of  a  terms,  a  is  the  first  term  and  B  the  ratio  of  any  term 
to  the  preceding  term. 

The  observed  relative  yields  are  shown  together  with  the  calculated  yield  for 
diminishing  returns  in  table  27. 


TABLE  27 
Diminishing  Physical  Returns  from  Increasing  Applications  of  Nitrogen 


Nitrogen  in  pounds  per  acre 

Relative  yields 

Observed 

Theoretical 

50 

76 
94 
106 
114 
128 
135 
143 

76  5 

100 

92  5 

150 

105  5 

200 

117. 

250 

127. 

300 

135.5 

350 

143. 

This  relationship  is  illustrated  in  Figure  3. 


9  Application  of  the  law  of  diminishing  returns  to  some  fertilizer  and  feed  data, 
by  Spillman,  W.  J.,  Jr.    Farm  Econ.,  vol.  5,  no.  1,  pp.  36-52.    January,  1923. 


